1. Field of the Invention
The present invention is generally related to communication systems and, more specifically, to private branch exchange systems including key systems, and methods for operating same.
2. Brief Description of the Pertinent Prior Art
A modern private branch exchange (PBX) system, in addition to facilitating communication among its lines and extensions must be capable of providing a number of special user services. For example, a PBX system must be capable of generating and routing, to each line and extension, a variety of standard and non-standard informational and supervisory tonal signals, such as, dial tone, audible ringing signal, etc., to provide each user with individualized call-status information. Furthermore, recently, it has also become desirable to provide such features as "music on hold", "voice mail", "station message detail recording" (SMDR), and "call forwarding." Also, given the needs of the modern office, the PBX system must have a conferencing feature service for temporarily connecting a plurality of PBX system lines and/or extensions so that they may converse and/or exchange data simultaneously.
Conventional PBX systems exist which are capable of providing the foregoing features. Typically, however, in order to be able to provide each of these special and particularly desirable features, conventional PBX systems incorporate application specific, dedicated hardware. For example, separate custom-made hardware modules are used in conventional systems to facilitate conference calling, generate tonal information signals and to distribute these signals to the plurality users (usually by means of a separate bus network), and to store and retrieve data (including voice messages in "voice mail" systems).
A number of disadvantages result from the use of application specific circuitry to provide special user services in conventional systems. For example, the custom hardware used to generate tonal signals is only capable of producing a limited number of frequencies and amplitudes (power levels), and therefore, the tones which may be provided are invariably fixed depending on the specific type of custom hardware used within the system. This is especially unfortunate given the nature of today's telecommunications standards and global economy. Signalling frequency and amplitude standards differ from country to country, necessitating modifications of tonal signals to conform to each country's communications standards. Also, in order to effect these changes, significant and costly modifications must be made to the system's hardware, in order for the PBX system to be able to conform to these different telecommunication standards. In certain cases, the required modifications may be so extensive as to necessitate substituting an entirely different PBX system for the one currently in use.
Serious signal attenuation problems also plague conventional systems. For example, in a conventional system, the custom hardware used to generate tonal signals is connected, in parallel, to each of the output ports (via a distribution network). This arrangement causes the signal power distributed to each of the output ports to decrease proportionately along with the addition of each new output port and/or output line, necessitating the use of amplification and/or signal repeater circuitry distributed throughout the system.
Further problems may result when "voice mail" features are incorporated into a typical PBX system. Conventionally, the circuitry used to provide "voice mail" features comprise separate and distinct systems from the main PBX system. These separate systems are interfaced to the main PBX system via analog telephone lines and communicate with each other using one or more predetermined, standard signaling protocols (which may be, for example, combinations of hook-flashes, dual tone multiple frequency signals, or other protocols). Thus, each of the systems (the "voice mail" and main PBX systems) must be provisioned with necessary hardware to communicate using combinations of appropriate timing instructions and/or other protocol commands so as to synchronize their operations. This can result in the profound disadvantage that, if synchronization is lost between the separate systems, the overall PBX system will no longer function properly. Additionally, during initialization, the user of conventional "voice mail" systems may experience long periods of delay, since the communications protocol must be established and initialized among the separate systems prior to the beginning of data transfer to the voice mail system. Also, since each of the systems is distinct from the other systems, each system must have its own individual control circuitry and administrative/control information data bases, and therefore, must be individually administered and maintained. These two problems (the requirement for main PBX and voice mail system synchronization, and separate, distinct administrative/control means for each system) serve to significantly increase the overall system's maintenance costs, required administrative down-time, and electronic complexity.
Finally, PBX "call forwarding" features currently available are quite limited in their capabilities. Typical "call forwarding" schemes merely re-route, after a predetermined amount of time, an incoming call from one line or extension of the PBX system to another predetermined extension. This can result in, inter alia, two major disadvantages. First, inconveniently, if the second extension is not answered, then, the caller must attempt to contact the desired party later by making a second call to the PBX system. Second, since a conventional PBX system's "call forwarding" circuitry may only redirect an incoming call from one PBX extension to another extension within the system, and not to an outside line remote from the system, individuals who usually would be at a given PBX extension (for example, in the workplace), but are currently at a location remote therefrom (for example, at home or commuting) cannot be reached by the PBX system.
Examples of conventional communications and/or private branch exchange systems include U.S. Pat. No. 4,215,247, issued to Lambert; U.S. Pat. No. 4,280,216, issued to Zeitrag; U.S. Pat. No. 4,566,096, issued to Sarson, et al; U.S. Pat. No. 4,701,948, issued to Molnar; U.S. Pat. No. 4,964,157, issued to Aoshima; U.S. Pat. No. 4,480,330, issued to Magnusson, et al; and U.S. Pat. No. 4,446,807, issued to Kerr, et al.